The present invention relates to a color composing optical system which composes colors of modulated light beams each emitted from a light valve for each color, to be projected in a full-color projecting apparatus, etc.
1. Related Art
FIG. 1 shows a schematic arrangement of a projecting apparatus which performs three-color decomposition and composition by use of a cross dichroic prism.
A white light emitted from a white light source 210 enters a polarizing beam splitter 220 and is polarized and separated by a polarization/separation portion of the polarizing beam splitter. The polarized light reflected and emitted (S-polarized light) is incident on the above-mentioned cross dichroic prism 230.
A red light component of the incident light is reflected by a red light reflecting dichroic film 230R inside the cross dichroic prism 230, so as to enter a reflection-type liquid crystal light valve 240R as a read light. Then, only a blue light component of this light is reflected by a blue light reflecting dichroic film 230B, so as to enter a liquid crystal light valve 240B for blue light as a read light.
A green light component transmitted through the above-mentioned red light and blue light reflecting dichroic films of the cross dichroic prism 230 is propagated as it is and is emitted from the cross dichroic prism 230, so as to enter a green light liquid crystal light valve 240G.
Each color light component entering the reflection-type liquid crystal light valve for the color is modulated by a signal recording light or an electric signal for each color (neither of which is shown in the figure) and is reflected and emitted from the light valve, so as to enter the cross dichroic prism 230 with the same optical axis as that of the above-mentioned incident light.
Then, the three light components are composed by the dichroic film 230R and the dichroic film 230B inside the cross dichroic prism 230, and emitted from the dichroic prism to enter the polarizing beam splitter 230, in which only a modulated light is analyzed. The modulated light is transmitted through the polarizing beam splitter and emitted to be projected onto a screen 260 through a projecting lens 250.
The cross dichroic prism 230 is comprised of, as shown in FIG. 2 for example, transparent optical glass members 231, 232, 233, 234 each taking the shape of a right-angled isosceles triangular prism. The sides of these isosceles triangular prisms are stuck together with predetermined dichroic films therebetween.
An arrangement of components of the conventional cross dichroic prism 230 is shown in FIG. 3. A member 231 and another member 233 are provided with the red light reflecting dichroic films each formed only on one side of two crossing four-cornered sides thereof, while a member 232 and a member 234 are provided with blue light reflecting dichroic films formed only on one side of two crossing four-cornered sides thereof, in the same manner.
The cross dichroic prism 230 mentioned above is produced by cementing the members shown in FIG. 3 together with an adhesive agent sandwiched therebetween.
Since it becomes difficult to maintain a predetermined accuracy in cementing when the four members are cemented together at one time, such a method of producing the cross dichroic prism is usually employed in which the members 231 and 232 are cemented together in advance to form a first connected member, while the members 233 and 234 are cemented together to form a second connected member, and finally the first connected member and the second connected member are cemented toghther to form the cross dichroic prism.
It should be noted that an arrangement in which one cross dichroic prism is used both for color decomposition and composition was employed for description of the projecting apparatus as the conventional device. However, there is present another projecting apparatus which performs color decomposition by use of a plane dichroic mirror and uses a cross dichroic prism only for color composition.
In the conventional projecting apparatus described above, there arise a problem that a projected image partly becomes a double image, or the image becomes blurred.
The present inventors have intensely examined the conventional projecting apparatus and found, as a result, the above problems are caused by different refractive indexes of the glass materials of the prisms for constituting the cross dichroic prism used for the three-color composition inside the apparatus.
More specifically, the refractive indexes of the glass prisms as the constituent members are slightly different from each other, so that if, for example, parallel light beams are incident on the optical axis, these light beams are supposed to be emitted in parallel to the optical axis in the same manner. However, if the light beams are emitted not in parallel to the optical axis due to the different refractive indexes of the constituent members of the prism through which the light beams are transmitted, it brings about the above-mentioned problems. Particularly, the above-mentioned cross dichroic prism is arranged to have four right-angled isosceles triangular prisms which are jointed at the right angle portions thereof to form a prism which is square in cross section, so that the light beams propagated in parallel with the optical axis have to be transmitted through the three prism members. As a result, it can be said that a difference in the refractive index among these prism members brings about a large influence.
In order to solve these problems, it is necessary to substantially remove a difference in the refractive index among the four prism members which constitutes the cross dichroic prism, which, however, is not easy.
Another problem of the cross dichroic prism for causing the above-mentioned problems is brought about by the structure of a dichroic film.
More specifically, in order to have the function as a color filter, the dichroic films of the cross dichroic prism are, seen in cross section thereof, provided on the both sides at the central portion and have a difference in level and an angle therebetween. These difference in level and angle cause a double or blurred image.
In the conventional dichroic prism, particularly the above-mentioned difference level is inevitably generated due to the thickness of the adhesive agent and can not be avoided so long that the adhesive agent has a finite thickness. In other words, a double or blurred image caused by it can not be prevented.
The above-mentioned two drawbacks are especially important in recent projecting apparatuses for projecting a large image, in which a double image becomes conspicuous in proportion to the size of a projected image.